The index of a function in the WasmModule data structure is offset by
the number of imported functions in the module. The {DecodeFunctionBody}
function of the module decoder, however, requires the function index
without this offset. The streaming processor mixed up these two ranges
of function indices. This is fixed in this CL.

R=clemensh@chromium.org

Bug: chromium:781507
Change-Id: Ie3e0c4703b06ecb923c98ffb961844915323197c
Reviewed-on: https://chromium-review.googlesource.com/776680
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49523}